JP6624959B2 - Continuous casting apparatus and slab manufacturing method - Google Patents

Description

  The present disclosure relates to a continuous casting apparatus and a method for manufacturing a slab.

  In a curved or vertical bending type continuous casting apparatus, a bent slab is straightened in a straightening band in a substantially straight line. Along with the correction in the correction band, tensile strain is generated on the L surface side, which is the inside of the curve of the slab. This tensile strain induces cracks on the surface of the slab. It is known that such surface cracks are more likely to occur when straightening is performed in a temperature region where the γ grain boundaries are embrittled and hot ductility is reduced.

  In order to reduce the surface cracks of the slab due to the correction, Patent Literature 1 proposes a technique in which the surface temperature of the slab is temporarily cooled to the A3 transformation point or lower, and then reheated to correct it. In this technique, the surface of the slab has a solidified structure in which ferrite and pearlite in which austenite grain boundaries (γ grain boundaries) are indistinct are mixed to suppress generation of cracks.

JP-A-2002-86252

  Patent Literature 1 proposes cooling the surface of a slab to an appropriate temperature range by setting the amount of water used for secondary cooling to a predetermined range. Thereby, it is considered that the solidified structure on the surface is refined and the occurrence of surface cracks is suppressed. However, according to the study of the present inventors, it has been found that even if the amount of water for secondary cooling is controlled, cracks may be locally generated on the surface of the slab with correction. Was.

  Therefore, an object of the present invention is to provide, in one aspect, a continuous casting apparatus capable of sufficiently suppressing the occurrence of surface cracks. Another object of the present invention is to provide a method for manufacturing a slab that can manufacture a slab having sufficiently reduced surface cracks.

  The present invention, in one aspect, is a continuous casting apparatus including a secondary cooling zone that cools a slab extracted from a mold, and a straightening zone that straightens the slab downstream of the secondary cooling zone. Between the secondary cooling zone and the straightening zone, a gas discharge section for blowing gas to the surface on the L-plane side of the slab, and supplying water to the gas-sprayed surface to change the surface of the slab to the A1 transformation point. The present invention provides a continuous casting apparatus including a water supply unit for cooling.

  In the above continuous casting apparatus, the gas discharge unit blows the gas onto the surface on the L-plane side of the slab to remove scales such as oxides that adhere to the surface of the slab and hinder cooling. This scale removal suppresses local variation in cooling temperature on the surface of the slab during cooling with water supplied from the water supply unit. As a result, the entire surface of the slab is cooled below the A1 transformation point to be miniaturized, and the occurrence of local surface cracks can be suppressed.

  The gas discharge unit is configured to blow gas to a corner on the L-side surface of the slab, and the water supply unit is configured to supply water to a corner on the L-side surface of the slab. It may be. Surface cracks are more likely to occur at the corners of the slab during straightening than at other parts. For this reason, if the gas is blown to the corners on the surface on the L-plane side to remove the scale and water is supplied to the portions for cooling, the solidified structure on the surface of the corners can be more uniformly refined. . On the other hand, excessive cooling is avoided at the center between the two corners on the L-surface side of the slab, and the heat can be smoothly recovered before straightening.

  The surface on the L-plane side of the slab is cooled by the water supply unit and then reheated to the A3 transformation point or higher by heat transfer from the inside of the slab. As a result, the surface cracks of the slab that occur during straightening can be further reduced.

  In another aspect, the present invention provides a first cooling step of secondary cooling a slab in a secondary cooling zone, a gas discharging step of blowing a gas onto the L-side surface of the secondary cooled slab, A cooling step in which the surface of the slab is cooled below the A1 transformation point by supplying water to the surface sprayed with, and the surface on the L-plane side of the slab cooled in the second cooling step is raised above the A3 transformation point. Provided is a method for manufacturing a cast piece, comprising: a recuperating step of recuperating; and a correcting step of correcting the cast piece after the recuperating step.

  In the above-described method for manufacturing a slab, a scale such as an oxide that adheres to the surface of the slab and hinders cooling is removed by a gas discharging step of blowing a gas onto the L-side surface of the slab. Therefore, at the time of cooling with water in the second cooling step, local variations in cooling temperature on the surface of the slab are suppressed. As a result, the entire surface of the slab is cooled below the A1 transformation point to be miniaturized, and the occurrence of local surface cracks can be suppressed.

  The manufacturing method has a recuperation step between the second cooling step and the straightening step. In this heat recovery step, the surface of the slab on the L-plane side is reheated to the A3 transformation point or higher by heat transfer from the inside of the slab. As a result, the surface cracks of the slab that occur during straightening can be further reduced.

  In the gas discharging step, a gas may be blown to a corner on the L-side surface of the slab, and in the second cooling step, water may be supplied to a corner on the L-side surface of the slab. Surface cracks are more likely to occur at the corners of the slab during straightening than at other parts. For this reason, if the gas is blown to the corners on the surface on the L-plane side to remove the scale and water is supplied to the portions for cooling, the solidified structure on the surface of the corners can be more uniformly refined. . On the other hand, excessive cooling is avoided at the center between the two corners on the L-side surface of the slab, and the heat can be smoothly recovered before straightening.

  The present invention, in one aspect, can provide a continuous casting apparatus capable of sufficiently suppressing the occurrence of surface cracks. The present invention, in another aspect, can provide a method for manufacturing a slab that can manufacture a slab having sufficiently reduced surface cracks.

FIG. 1 is a view schematically showing a continuous casting apparatus according to one embodiment of the present invention. FIG. 2 is a graph showing an example of the behavior of the temperature of the surface on the L-plane side of the slab when the continuous casting apparatus according to one embodiment of the present invention is used. FIG. 3 is a diagram illustrating an example of a gas discharge unit and a water supply unit of the continuous casting device according to one embodiment of the present invention. FIG. 4 is a diagram showing another example of the gas discharge unit and the water supply unit of the continuous casting device according to one embodiment of the present invention. FIG. 5 is an enlarged cross-sectional view schematically illustrating an example of a solidified structure near the surface of a slab manufactured using the continuous casting apparatus according to one embodiment of the present invention. FIG. 6 is an enlarged cross-sectional view schematically showing a solidified structure near the surface of a slab produced using a conventional continuous casting apparatus.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings as necessary. However, the following embodiments are examples for describing the present invention, and are not intended to limit the present invention to the following contents. In the description, the same elements or elements having the same functions will be denoted by the same reference symbols, and duplicate description will be omitted in some cases. Unless otherwise specified, the positional relationship such as up, down, left, and right is based on the positional relationship shown in the drawings. Further, the dimensional ratios in the drawings are not limited to the illustrated ratios.

  FIG. 1 is a diagram schematically illustrating a curved continuous casting apparatus 100 according to the present embodiment. The continuous casting apparatus 100 includes a ladle 102 filled with molten steel 101, a tundish 105 provided below the ladle 102, and storing the molten steel 101 passing from the bottom of the ladle 102 through the cylindrical hole of the ladle nozzle 103. , A mold 104 provided below the tundish 105 and a slab support 110.

  The slab support 110 includes a secondary cooling zone 14 through which the slab 10 drawn from the mold 104 flows downward, an arc portion 15 for sending the curved slab 10 to the correction zone 16, and a curved slab 10. And a horizontal band 17 for conveying the slab 10 in the horizontal direction. Each of the secondary cooling zone 14 and the arc portion 15 is provided with a support roll 20 that is in contact with the slab 10 on the outer peripheral surface so as to oppose the slab 10 in the thickness direction.

  The support roll 20 has a rotating shaft extending along the width direction of the slab 10, and is configured to flow the slab 10 downstream while rotating about the rotating shaft. The slab 10 is guided by the support roll 20, advances through the secondary cooling zone 14 and the arc portion 15, and enters the straightening zone 16.

  A correction band 16 is provided downstream of the arc portion 15. The straightening belt 16 has a plurality of straightening rolls 22 that are in contact with the slab 10 on the outer peripheral surface so as to face the slab 10 across the thickness direction. The slab 10 that has passed through the arc portion 15 is bent back by the plurality of straightening rolls 22 and straightened into a substantially linear shape. The straightening roll 22, like the support roll 20, has a rotation axis extending along the width direction of the cast piece 10, and presses and corrects the cast piece 10 while rotating about the rotation axis. . A support roll 24 is provided on the horizontal band 17 disposed downstream of the correction band 16. The slab 10 straightened by the straightening band 16 is drawn out in the horizontal direction by the horizontal band 17.

  The slab 10 may be a bloom, billet or slab. The bloom may have a cross-sectional area of, for example, 200 mm × 200 mm or more when viewed in a cross section when cut along the thickness direction. The billet may have a cross-sectional area of, for example, 50 mm × 50 mm or more and less than 200 mm × 200 mm when viewed in the cross section. The slab may have a short side of 50 mm or more and a long side of 800 to 3000 mm when viewed in the cross section.

  The molten steel 101 in the tundish 105 is primarily cooled in the mold 104, and the surface is solidified. As a result, a so-called solidified shell 11 is formed on the surface of the slab 10. On the other hand, the slab 10 has a molten portion (unsolidified portion) 12 inside a solidified shell 11. The temperature of the surface of the slab 10 is measured by, for example, a radiation thermometer. The temperature of the surface of the slab 10 may be obtained by calculation by solidification heat transfer analysis.

  The slab 10 primarily cooled by the mold 104 is further cooled in the secondary cooling zone 14. As a result, the solidified shell 11 grows, and the molten portion 12 shrinks. The secondary cooling zone 14 is provided with a nozzle 30 for discharging cooling water to the surface of the slab 10. The nozzle 30 discharges cooling water to four surfaces of the slab 10, the L surface, the F surface, and the S surface (side surface). This causes the solidified shell to grow on both surfaces. The surface of the slab 10 is cooled to, for example, 900 to 1000 ° C. by the action of the cooling water discharged from the nozzle 30. By cooling in this way, the solidified shell 11 grows and scale is generated on the surface of the slab 10.

  FIG. 2 is a graph showing an example of the behavior of the temperature of the surface on the L-plane side of the slab 10. The horizontal axis in FIG. 2 is the distance from the mold 104, and the vertical axis is the temperature. The section R1 in FIG. 2 corresponds to a section for secondary cooling. By the secondary cooling, the surface of the slab 10 is cooled to, for example, 960 ° C. After the secondary cooling, the slab 10 is reheated to, for example, 980 ° C. In addition, the above-mentioned cooling temperature and recuperation temperature can be appropriately adjusted according to casting conditions.

  Returning to FIG. 1, the slab 10 cooled in the secondary cooling zone 14 enters the arc portion 15. In the present specification, the arc portion 15 refers to a region between the secondary cooling zone 14 and the correction zone 16. In the arc portion 15, the slab 10 may be reheated as shown in FIG.

  The arc portion 15 includes, from the upstream side, a gas discharge unit 40 that blows gas onto the L-side surface of the slab 10 and a water supply unit 50 that supplies cooling water to the L-side surface of the slab 10. Provided. The L surface of the slab 10 is the inner peripheral surface of the curved slab 10. The outer peripheral surface opposite to the L surface is sometimes referred to as an F surface (fixed surface).

  In the correction band 16 arranged on the downstream side of the arc portion 15, a large tensile strain is generated on the L surface side of the slab 10. For this reason, a surface crack accompanying correction is likely to occur on the L side. In order to reduce this surface crack, the gas discharge unit 40 and the water supply unit 50 are configured to supply gas and cooling water to the surface on the L-plane side, respectively. However, the gas discharge unit 40 and the water supply unit 50 may be configured to supply gas and cooling water to the surface on the F surface as well as the L surface. From the viewpoint of simplifying the device configuration and increasing the efficiency of the production of cast slabs, the gas discharge unit 40 and the water supply unit 50 may be configured to supply gas and cooling water only to the surface on the L side. preferable.

  The gas discharge unit 40 may adopt a configuration in which gas is blown onto the L surface side of the slab 10 to remove scale attached to the surface of the slab 10 on the L surface side. Any structure can be used as long as the scale can be removed, and the structure is not limited to a specific structure. As the gas, air or an inert gas can be used. As the scale, a foreign substance such as an oxide (slag) is used.

  FIG. 3 is a diagram schematically illustrating an example of the gas discharge unit 40. The gas discharge unit 40 is configured by a pair of nozzles that discharge gas. The gas discharge unit 40 is configured to discharge gas toward the corner 19 on the surface 10 a on the L-plane side of the slab 10. For example, the gas may be discharged toward the corner portion 19 by directing the discharge port of the nozzle toward the corner portion 19 on the L-side surface 10a. In addition, the corner part 19 is a part including the corner of the slab 10 and its vicinity. In FIG. 3, the corner 19 on the surface 10a has a length L1.

  By providing the gas discharge portion 40 for discharging gas toward the corner portion 19 on the L-side surface 10a of the slab 10, the scale attached to the corner portion 19-side surface 10a of the L-side surface 10a is reduced. It can be removed efficiently. Thus, insufficient cooling of the surface 10a of the slab 10 by the water supply unit 50 due to the adhesion of the scale can be sufficiently avoided.

  There is no particular limitation on the area of the surface 10a with which the gas discharged from the gas discharge unit 40 comes into contact. However, when viewed in a cross section of the slab 10 as shown in FIG. May be discharged. L1 corresponds to the width (each width of the surface 10a) of the region where the gas discharged from the gas discharge unit 40 collides in the cross section of the slab 10. Although only a pair of nozzles are shown in FIG. 3, the gas discharge unit 40 may be configured by a plurality of nozzles arranged at predetermined intervals along the longitudinal direction of the slab 10.

  The discharge pressure of the gas discharged from the gas discharge unit 40 is not particularly limited. From the viewpoint of suppressing supercooling while sufficiently removing the scale, the discharge pressure of the gas from the gas discharge unit 40 may be, for example, 0.3 to 0.5 MPa.

  A discharge unit for discharging gas to the secondary cooling zone 14 is separately provided together with the gas discharge unit 40, and before secondary cooling with water, the gas is blown onto the L-side surface of the slab 10 to remove scale. You may. However, since the growth of the solidified shell 11 has not sufficiently progressed in the secondary cooling zone 14, the scale deposited after the secondary cooling is removed at the gas discharge unit 40.

  FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 1, and is a diagram schematically illustrating an example of the water supply unit 50. The water supply unit 50 includes a pair of nozzles that discharge water. The water supply unit 50 is configured to discharge water toward the corner 19 on the surface 10 a on the L-plane side of the slab 10. For example, water may collide with the corner 19 on the surface 10a by directing the outlet of the nozzle to the corner 19 on the surface 10a. The water may be discharged from the water supply unit 50 in a spray form. Although only a pair of nozzles are shown in FIG. 3, the water supply unit 50 may be configured by a plurality of nozzles arranged at predetermined intervals along the longitudinal direction of the slab 10. The nozzle of the water supply unit 50 may be a double tube nozzle.

  By using the water supply unit 50 that supplies water toward the corner 19 of the L-side surface 10a of the slab 10, the surface 10a on the corner 19 side can be efficiently cooled. Thereby, the solidified structure of the surface 10a on the corner portion 19 side can be sufficiently refined. Further, it is possible to avoid excessive cooling of the surface 10a at the central portion between the corner portions 19 where cracks are less likely to occur than at the corner portions 19. This makes it possible to smoothly reheat the slab 10 before straightening, and it is possible to further suppress the surface cracks and further improve the efficiency of the slab manufacturing process.

  There is no particular limitation on the region where the water discharged from the water supply unit 50 contacts, but when viewed in a cross section of the slab 10 as shown in FIG. 3, the water is discharged such that L1 is 50 to 150 mm, for example. May be done. L1 corresponds to the width (the width of the surface 10a) of the region where the cooling water discharged from the water supply unit 50 collides in the cross section of the slab 10. The supply amount of the cooling water from the water supply unit 50 is appropriately set according to casting conditions such as the type of steel and the casting speed.

  The surface 10 a of the slab 10 is cooled to, for example, the A1 transformation point or lower by the water supplied from the water supply unit 50. As a result, the solidification structure near the surface 10a on the L-plane side of the slab 10 is transformed and refined. The surface 10a on the L-plane side of the slab 10 may be cooled to, for example, 700 ° C. or lower, preferably 650 ° C. or lower. By cooling to such a temperature, the refinement of the solidified structure in the vicinity of the surface 10a of the slab 10 is further promoted. Thereby, the occurrence of cracks on the surface 10a on the L-plane side of the slab 10 can be sufficiently suppressed.

  The section R2 in FIG. 2 corresponds to a cooling section using cooling water supplied from the water supply unit 50. By supplying the cooling water from the water supply unit 50, the surface 10a of the slab 10 is cooled to about 600 ° C.

  In the arc portion 15, the surface 10 a of the slab 10 is cooled by the cooling water from the water supply unit 50, and then is regained by the heat of the melting portion 12 of the slab 10. For example, cracks that occur during correction in the correction band 16 can be further reduced by reheating at or above the A3 transformation point. In the example of FIG. 2, before being corrected, the surface 10a of the slab 10 is reheated to the A3 transformation point (850 ° C.) or higher. In order to reheat the slab 10, in addition to a method using heat from the molten portion 12 of the slab 10, an induction heater including a coil made of a conductor configured to generate an induction current in the slab 10 is used. You may go.

  FIG. 4 is a diagram schematically illustrating another example of the gas discharge unit provided in the continuous casting apparatus 100. The gas discharge unit 40A is configured by a nozzle that discharges a gas. Although only one nozzle is shown in FIG. 4, the gas discharge unit 40 </ b> A may be configured by a plurality of nozzles arranged at predetermined intervals along the longitudinal direction of the slab 10. The gas discharge unit 40A is configured to discharge gas toward the entire surface 10a on the L-plane side of the slab 10. The gas discharge unit 40A can remove the scale on the surface 10a on the L-plane side of the slab 10. The number of nozzles of the gas discharge unit 40A of FIG. 4 can be smaller than that of the gas discharge unit 40 of FIG. Therefore, the device configuration can be simplified.

  FIG. 4 is a diagram schematically illustrating another example of the water supply unit provided in the continuous casting apparatus 100. The water supply unit 50A is configured by a nozzle that discharges water. Although only one nozzle is shown in FIG. 4, the water supply unit 50 </ b> A may be configured by a plurality of nozzles arranged at predetermined intervals along the longitudinal direction of the slab 10. The water supply unit 50A is configured to discharge water toward the entire surface 10a of the slab 10. By the cooling water discharged from the water supply unit 50A, the surface 10a on the L side of the slab 10 is cooled to an A1 transformation point or lower. The water supply unit 50A of FIG. 4 can have a smaller number of nozzles than the water supply unit 50 of FIG. Therefore, the device configuration can be simplified. The A1 transformation point varies depending on the composition of the slab, but is generally about 700 ° C. The slab 10 is, for example, a low alloy steel containing alloy elements such as Nb, V, Ni, and Cu.

  The slab 10 cooled in the arc portion 15 enters the straightening zone 16 after being reheated as required. In the straightening belt 16, the slab 10 is pressed down by a straightening roll 22 and straightened in a substantially straight line. The corrected slab 10 enters the horizontal band 17. In the horizontal band 17, the slab 10 may be reduced or lightly reduced.

  FIG. 5 is an enlarged cross-sectional view schematically illustrating an example of a solidified structure near the surface 10a of the slab 10 manufactured using the continuous casting apparatus 100. The slab 10 has, from the surface 10a side, a transformed portion 10A having a finely solidified structure, and an untransformed portion 10B composed of crystal grains larger than the transformed portion 10A inside the transformed portion 10A. . FIG. 5 shows an enlarged region E including the boundary between the transformed portion 10A and the untransformed portion 10B. Transformation part 10A is a region where the γ grain boundary disappears due to the formation of ferrite at the γ grain boundary in a granular form. On the other hand, the untransformed portion 10B is a region where ferrite is formed in a film shape along the γ grain boundary, and is a region where a clear γ grain boundary is recognized.

  The grain size of the crystal grains in transformed portion 10A is, for example, 10 μm or less. On the other hand, in the untransformed portion 10B, the grain size of the crystal grain partitioned by the γ grain boundary is, for example, 100 μm or more. The difference between the structure of the transformed portion 10A and the structure of the untransformed portion 10B can be confirmed by polishing the sample cut from the slab, performing 5% nital corrosion, and then observing it with an electron microscope or an optical microscope.

  The surface 10a on the L-plane side of the slab 10 is composed of the transformed portion 10A, and the untransformed portion 10B is not exposed. For this reason, when the slab 10 is straightened in the straightening band 16, the occurrence of surface cracks can be sufficiently suppressed. Transformation section 10A has a thickness of, for example, 1 to 30 mm. In the present embodiment, the untransformed rate on the surface 10a on the L-plane side of the slab 10 can be 10% or less, preferably 5% or less. The untransformed rate is determined as a ratio of the length of the portion where the transformed portion is not formed to the entire length of the slab 10 on the surface 10a when viewed along a cut surface along the line direction.

  FIG. 6 is an enlarged cross-sectional view schematically showing a solidified structure near the surface 10c on the L-plane side of the slab 10 manufactured by using the conventional continuous casting apparatus. The untransformed portion 10B is exposed on the surface 10c on the L-plane side of the slab 10. The exposed untransformed portion 10B is caused by insufficient cooling caused by the scale attached to the surface 10c of the slab 10. Since the untransformed portion 10B is more brittle than the transformed portion 10A, a surface crack 200 is generated in the untransformed portion 10B exposed on the surface 10c during the correction.

  The method for manufacturing the slab 10 of the present embodiment can be performed using the continuous casting apparatus 100 shown in FIG. In the method of manufacturing a slab according to the present embodiment, a first cooling step of secondary cooling the slab 10 in the secondary cooling zone 14 and blowing gas to the L-side surface 10 a of the secondary cooled slab 10 are performed. A gas discharging step, a second cooling step of supplying water to the gas-sprayed surface 10a to cool the surface 10a of the slab 10 below the A1 transformation point, and the L of the slab 10 cooled in the second cooling step. It has a recuperation step of recuperating the surface 10a on the front side to the A3 transformation point or higher, and a correction step of correcting the slab 10 after the recuperation step.

  In the first cooling step, the slab 10 is cooled by cooling water discharged from the nozzle 30 provided in the secondary cooling zone 14. The temperature of the surface of the cast slab 10 after cooling is, for example, 900 to 1000 ° C. Prior to the first cooling step, a step of forming the solidified shell 11 by primary cooling the surface of the molten steel 101 in the mold 104 to about 1200 ° C. (preliminary cooling step) is performed.

  In the gas discharging step, gas is blown onto the L-side surface 10a of the secondary cooled slab 10 using the gas discharging unit 40 to remove scale adhering to the surface 10a. In the gas discharging step, as shown in FIG. 3, gas may be blown toward the corner portion 19 on the surface 10 a of the slab 10 to remove scale mainly attached to the surface 10 a of the corner portion 19. Thereby, in the second cooling step described later, it is possible to sufficiently cool the corner portions 19 on the surface 10a where the surface cracks easily occur. In the gas discharging step, as shown in FIG. 4, a gas discharging unit 40A configured to discharge gas toward the entire surface 10a of the slab 10 may be used.

  In the second cooling step, water is supplied to the surface 10a onto which the gas has been sprayed by using the water supply unit 50 to cool the surface 10a of the slab 10 to the A1 transformation point or lower. As a result, the solidified structure near the surface 10a on the L-plane side of the slab 10 is transformed and refined. In the second cooling step, the surface on the L side of the slab 10 may be cooled to, for example, 700 ° C. or less, preferably 650 ° C. or less. Thereby, the solidification structure of the surface 10a on the L-plane side of the slab 10 can be sufficiently refined.

  In the second cooling step, as shown in FIG. 3, cooling water may be supplied to corners 19 on the surface 10 a of the slab 10. Thereby, the solidified structure of the surface 10a on the corner portion 19 side can be more reliably miniaturized. On the other hand, excessive cooling is avoided at the surface 10a in the center between the two corners 19 of the slab 10, and the slab 10 can be smoothly reheated before straightening. In the second cooling step, as shown in FIG. 4, a water supply unit 50A configured to discharge cooling water toward the entire surface 10a of the slab 10 may be used.

  The method for manufacturing the slab 10 includes a recuperation step between the second cooling step and the straightening step. In the heat recovery step, the surface 10a on the L side of the slab 10 is heated to 850 ° C. or higher. Thereby, generation of surface cracks on the surface 10a on the L-plane side of the slab 10 can be more sufficiently suppressed. The description of the continuous casting apparatus 100 described above is appropriately applied to the method of manufacturing the cast slab 10 of the present embodiment.

  As described above, the preferred embodiments of the present invention have been described, but the present invention is not limited to the above embodiments. For example, in the above-described embodiment, the gas and water are supplied from the gas discharge unit and the water supply unit only to the surface 10a on the L-plane side of the slab 10, but the present invention is not limited to this. For example, the gas discharge from the gas discharge unit is performed on both the L surface side and the F surface side which is the curved outer side of the slab, or the L surface, the F surface and the S surface which is the curved side surface of the slab, and includes It may be performed on all surfaces. The supply of water from the water supply unit may be performed on both the L surface and the F surface, or on all four surfaces including the L surface, the F surface, and the S surface. However, the supply of water from the water supply unit may be performed only to the surface on the L-plane side, from the viewpoint of smoothly recovering heat after cooling with water from the water supply unit. Further, from the viewpoint of simplifying the device configuration, the discharge of the gas from the gas discharge unit and the supply of the water from the water supply unit may be performed only on the surface on the L-plane side. The continuous casting apparatus is not limited to the curved type, but may be a vertical bending type. Also in the case of the vertical bending type continuous casting apparatus, the arc portion refers to a region on the downstream side of the secondary cooling zone, and the arc portion is disposed between the secondary cooling zone and the straightening zone.

  The content of the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

(Example 1)
A slab was manufactured using the continuous casting apparatus shown in FIG. As the gas discharge section and the water supply section, a pair of nozzles (inner diameter: 10 mm) as shown in FIG. 3 were arranged only on the L-face side of the slab. The nozzles of the gas discharge part and the water supply part were arranged at a position where the center line was 50 mm inward from the corner of the slab (the position where L1 in FIG. 3 was 100 mm). In addition, each nozzle was arranged such that the gas and water discharge ports were at a height of 100 mm from the upper surface of the slab.

Air was blown from the nozzles of the gas discharge unit arranged as described above at a discharge pressure of 0.5 MPa toward a corner on the L-side surface of the slab. Thereafter, water was sprayed and discharged to a corner portion on the surface on the L side of the slab to which air had been blown. The production conditions of the slab were as follows.
-Casting speed: 0.5 m / min-Slab size: 400 mm x 500 mm (thickness x width)

  After water was supplied from the water supply unit to cool the surface of the slab on the L-plane side to 600 ° C., the slab was reheated to about 870 ° C. Then, it was straightened in a straightening band to obtain a cast piece. The temperature change of the surface on the L side of the slab was as shown in FIG. The obtained slab was cut out to obtain a sample for evaluating a solidified structure. The evaluation of the solidification structure was performed on a cut surface perpendicular to the L plane along the line direction. The evaluation was performed visually near the L-side surface. The ratio of the length (L0) of the portion where the transformed portion was not formed to the predetermined length (L2 = 100 mm) along the line direction of the slab was calculated as the untransformed rate (L0 / L2 × 100). In addition, the number of surface cracks on the surface on the L-plane side at the corners was examined using an electron microscope, and the number of surface cracks per 10 mm in length was calculated. The number of surface cracks was counted only when the length in the depth direction was 50 μm or more. Table 1 shows the results.

(Comparative Example 1)
A slab was produced in the same manner as in Example 1 except that no gas was blown from the gas discharge portion. The cross section of the obtained slab was evaluated in the same manner as in Example 1. Table 1 shows the evaluation results.

  As shown in Table 1, in the slab of Example 1, the untransformed rate was 0. That is, as shown in FIG. 5, the entire surface of the slab on the L side was covered with the transformed part. For this reason, no surface cracks occurred. On the other hand, in the slab of Comparative Example 1, as shown in FIG. 6, the untransformed portion was exposed on the surface on the L-plane side of the slab. Then, a surface crack occurred in the untransformed portion exposed on the surface. From these results, it was confirmed that the surface cracks could be reduced by blowing gas onto the L-side surface of the slab and supplying and cooling water to the surface.

  ADVANTAGE OF THE INVENTION According to this indication, the continuous casting apparatus which can suppress generation | occurrence | production of a surface crack sufficiently can be provided. Further, according to the present disclosure, it is possible to provide a method of manufacturing a slab that can manufacture a slab in which surface cracks are sufficiently reduced.

  DESCRIPTION OF SYMBOLS 10 ... Cast piece, 10A ... Transformed part, 10B ... Untransformed part, 10a ... Surface, 11 ... Solidified shell, 12 ... Melted part, 14 ... Secondary cooling zone, 15 ... Arc part, 16 ... Straightening zone, 17 ... Horizontal Belt, 19: corner portion, 20, 24: support roll, 22: straightening roll, 30: nozzle, 40, 40A: gas discharge portion, 50, 50A: water supply portion, 100: continuous casting device, 101: molten steel, 102 ... Ladle, 103 ... Ladle nozzle, 104 ... Mold, 105 ... Tundish, 110 ... Slab support, 200 ... Surface crack.

Claims (5)

A secondary casting zone that cools the cast slab extracted from the mold, and a straightening zone that straightens the cast slab downstream of the secondary cooling zone, comprising:
Between the secondary cooling zone and the straightening zone,
A gas discharge unit that blows a gas on the surface on the L surface side, which is the inner peripheral surface of the curved slab,
A water supply unit for supplying water to the surface to which the gas has been blown to cool the surface of the slab to an A1 transformation point or lower. The gas discharge unit is configured to blow the gas to a corner portion of the surface of the L side of the slab,
The water supply unit configured to supply the water to the corner portion of the surface of the L side of the slab, a continuous casting apparatus according to claim 1.   3. The continuous casting apparatus according to claim 1, wherein the surface on the L surface side of the slab is cooled by the water supply unit and then reheated to an A3 transformation point or higher. 4. A method for producing a slab using a continuous casting apparatus,
A first cooling step of secondary cooling the slab in a secondary cooling zone,
Secondary cooling, a gas discharge step of blowing a gas to the surface on the L surface side, which is the inner peripheral surface of the curved slab,
A second cooling step of supplying water to the surface sprayed with the gas to cool the surface of the slab to an A1 transformation point or lower;
A recuperation step of recuperating the L-side surface of the slab cooled in the second cooling step to an A3 transformation point or higher;
And a straightening step of straightening the slab after the recuperation step. Wherein the gas discharge step, by blowing the gas to the corner portion of the surface of the L side of the slab,
In the second cooling step, the manufacturing method of the supplying the water in the corner portion in the surface of the L side, cast strip of claim 4 of the slab.

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